1. Field of the Invention
This invention relates to techniques for treating symptoms of peripheral vascular disease and organ ischemia, and more particularly relates to techniques for improving blood flow to ischemic limbs and organs using spinal cord or peripheral nerve stimulation.
2. Description of Related Art
A large number of humans suffer from peripheral vascular disease (PVD). PVD refers to a condition that adversely affects blood flow in the limbs. PVD may be caused by a number of disorders with the body such as arteriosclerosis (with or without diabetes), Raynaud's disease (idiopathic vasospasms), Buerger's disease (thromboangiitis obliterans) or embolic occlusive disease. PVD causes poor blood flow to the limbs of the patient thereby causing ischemic pain in the arms or legs. There are several surgical techniques that may restore compromised blood flow, such as revascularization procedures or angioplasty, but these might not be appropriate, they might have already failed, or the patient might prefer to try less invasive therapy. Since ischemic pain is usually not responsive to narcotics, the patient has no available pain relief and must endure the pain. Poor blood flow also means reduced oxygen that is carried in the blood stream to cells. As oxygen falls to very low levels, cell death or necrosis can develop, leading to nonhealing lesions, gangrene and amputation. Skin oxygen level measured by tanscutaneous oximetry (TcPO2) is above 40 mm Hg and usually over 50 mm Hg for most normal human beings. PVD patients have much lower skin oxygen levels. If it is below 20 mm Hg, lesions will not heal. Below 10 mm Hg, tissue may deteriorate. When the blood flow reaches very low levels, the feet may hurt especially when the patient is lying down (rest pain). In this case, the limb blood supply may become critical (critical limb ischemia). If there is no solution to this pain, the patient may request that the limb be amputated.
In earlier stages of PVD, some ischemic pain does not come at rest (rest pain), but rather when the patient exercises. Called claudication pain, this type of pain is created in muscles which cannot get enough oxygen. Claudication pain can be elicited by the most basic physical activity such as walking. The patient will walk less and less and the cardiovascular system will further deteriorate. The quality of life for the patient thereby diminishes.
Clinical researchers have found that spinal cord stimulation (SCS) may be used to help improve blood flow and alleviate ischemic pain in limbs caused by poor blood flow due to peripheral vascular disease (PVD). SCS may reduce pain via the well known Gate Control Theory proposed by Ronald Melzack and Patrick Wall in 1965. There are also three probable mechanisms, based on animal and clinical research from SCS for PVD, of how SCS can improve blood flow: (1) inhibition of excess sympathetic nervous activity due to pain; (2) further inhibition of sympathetic nervous background tone; and (3) release of vasoactive substances in the limb. Normally the sympathetic nervous system controls peripheral blood flow. The sympathetic nervous system is able to contract the small precapillary smooth muscle sphincters that adjust local blood flow. During times of stress or vigorous activity, there is high sympathetic tone, and blood is shunted from the distal limbs to large proximal muscles. Pain in the periphery can also increase the sympathetic outflow and cause such shunting. Ironically, in the elderly person with PVD, the pain of limb ischemia can trigger more sympathetic outflow, and further decrease blood supply to distal limb areas. Researchers have studied the role of the sympathetic system in producing ischemia using rats and have shown that interventions that inhibit the sympathetic outflow like drugs or sympathetic ganglia destruction (sympathectomy) can dramatically increase vasodilation in the periphery. They have also shown in rats that SCS can recruit A-delta nerve fibers in dorsal roots (normally carrying pain messages). Antidromic actions potentials in these fibers travel to the distal limb parts and release CGRP and NO, molecules which greatly assist local vasodilation. Both of these effects may be present in humans getting SCS for their PVD, and depending upon the precise parameters of stimulation, may explain the mechanism for the benefit of vasodilation.
Typically during SCS for ischemic feet or legs, an epidural lead (such as the Medtronic, Inc., PISCES.RTM. or RESUME.RTM. lead) is placed to stimulate the spinal cord near the midline of the dorsal columns at T8-L1. This stimulation provides strong paresthesia in the feet, and may inhibit sympathetic spinal centers in the distal part of the spinal cord or cause the release of vasoactive substances, as discussed above. For ischemic pain in the hands, a similar epidural lead is typically placed on the symptomatic side of the spinal cord around C5-C8. If hand ischemic pain is bilateral, a dual channel system (such as the Mattrix.RTM. system sold by Medtronic, Inc.) or two independent systems are used. Benefits of SCS beyond relief of ischemic pain have also been studied and used with clinical success, such as warming the feet, healing of small ulcers, increasing of walking distances and improving the chances to avoid amputation of parts of a foot or the entire foot.
For all humans, the degree of limb ischemia varies greatly during the day. For the PVD patient, it often reaches painful and dangerous levels (relative to tissue health). In endstage PVD. the ischemic pain is worse when lying down since gravity cannot help the blood flow. In earlier PVD, it begins with exercise due to muscles competing for blood flow. Ischemic pain may even increase in response to ingestion of certain foods or medicines. For instance, medicines that decrease heart output may also cause reduced blood flow in the extremities thereby increasing ischemic pain. In general, any activity or condition that increases the sympathetic nervous system output can also divert blood from extremities to more proximal muscles (fight or flee reflexes). For all of the above reasons, SCS may be an effective therapy to not only decrease pain, by also improve limb blood flow.
Present commercially available SCS systems, however, do not provide a technique for automatically varying levels of SCS to account for these changing daily conditions. Most commercially available SCS systems have continuous stimulation at one or two amplitudes for neuropathic pain, and perhaps automatically cycle "on" or "off", but not according to the degree of limb ischemia. U.S. Pat. No. 5,702,429 (King), assigned to Medtronic, Inc. of Minneapolis, Minn., provides an SCS system that uses closed loop feedback techniques to adjust the parameters of stimulation (voltage, pulse width, frequency, etc.) automatically so that the conscious sensation (called paresthesia) or the degree of pain relief can be kept constant, regardless of patient position or activity. However, this art is not designed to keep constant any of the beneficial vascular effects of SCS for the PVD patient.
It would be advantageous to be able to have the parameters of stimulation automatically adjusted to prevent uncomfortable or dangerous conditions from occurring. This would be useful during sleep, when a patient cannot adjust said parameters, and also during daily activities or patient positions, so that both ischemic pain relief and vascular improvement can be maintained optimally, without distraction of the patient.
Additionally, constant stimulation of the spinal cord may not be desirable since stimulation may not be necessary during times when ischemia has subsided, it may have bothersome or adverse side effects, or it may cause the body to build up a tolerance to the stimulation. With closed-loop feedback control, the patient would be more likely to rest better and to enjoy an active lifestyle, and not have to limit activities or medications. The patient may also be able to keep the amplitudes of stimulation higher, knowing that a situation will not develop which is unduly painfull or disadvantageous to health. Additionally, the patient may be able to walk and otherwise exercise more.
The present invention is therefore directed to overcoming the disadvantages of the foregoing SCS, and to provide a degree of vascular improvement (proportional to the degree of ischemia reduction) that is otherwise unobtainable.